1. Field of the Invention
The present invention relates to a light receiver applied to an optical communication system using different communication protocols and, more particularly, to a light receiver capable of detecting a signal loss in an optical signal by following the receiving sensitivity.
2. Background Art
In an optical communication system, information such as a speech, a piece of electronic mail or electronic data including character and image information, typified by data transmitted in the Internet, is encoded into an optical signal in accordance with a frame system determined in a particular communication protocol to be transmitted through an optical fiber. A light receiver has the function of converting an encoded optical signal into an electrical signal.
FIG. 9 is a diagram showing the configuration of a conventional light receiver. As shown in FIG. 9, the light receiver is connected to an optical fiber 1 and has a photoelectric conversion circuit 2, an electric amplifier 3, and an optical signal loss detection circuit 4. The optical signal loss detection circuit 4 has a comparator 41 and a signal detection circuit (peak detection circuit) 42. The signal detection circuit 42 is a circuit for detecting a signal component in the electric amplifier 3. More specifically, the signal detection circuit 42 uses a peak detection output from a signal component. The comparator 41 compares a signal component detected by the signal detection circuit and a fixed threshold value given from the outside and outputs the result of comparison.
The operation of the conventional light receiver will be described. The photoelectric conversion circuit 2 first converts an optical signal transmitted through the optical fiber 1 into an electrical signal. The electric amplifier 3 then amplifies the converted electrical signal to a signal amplitude discriminable in a stage following the light receiver. The optical signal loss detection circuit 4 compares the electrical amplitude of the output signal from the electric amplifier 3 with the predetermined threshold value and determines whether or not the level of the optical signal input to the light receiver is larger than the predetermined threshold value. The optical signal loss detection circuit 4 outputs a digital signal based on the yes/no determination with respect to the threshold value.
The optical signal loss detection circuit 4 is used to detect an optical signal break. In an electrical signal processing block following the light receiver, detection can be performed by using the result of this optical signal loss detection as to whether or not there is any circuit abnormality, for example, due to a break in the optical fiber transmission line constituting a main circuit of the optical communication system and whether or not there is any abnormality in the operation on the transmitting side, for example, due to a reduction in optical output from a light transmitter. For example, in a situation where a circuit abnormality has occurred, for example, due to a break in the optical fiber transmission line, the above-described arrangement enables execution of a circuit changing operation in the electrical signal processing block following the light receiver to avoid a communication abnormality in a comparatively short time. Thus, the optical signal loss detection function of the light receiver is useful in maintenance and management of the optical communication system.
However, different communication protocols coexist in a communication system from an optical submarine cable, an intercity trunk network or the like to subscriber networks in buildings for example. More specifically, in circuits typified by a trunk system, encoding methods called ITU-T (International Telecommunication Union-Telecommunication sector), SDH (Synchronous Digital Hierarchy) in accordance with the Bellcore international standard, and SONET (Synchronous Optical Network) are used as communication protocols. In SDH and SONET, frame formats are determined in accordance with unique encoding methods and transmission rates corresponding to multiples of 4: 155.52 Megabit per second (Mbps), 622.08 Mbps, 2.48832 Gbps and 9.95328 Gbps are determined according to the circuit capacity.
For circuits typified by a local area network (LAN) provided as a subscriber system, a frame system called Ethernet (trademark) in conformity with the IEEE802.3 internal standard exists. In the Ethernet (trademark) frame system, a frame system in conformity with a unique encoding method is also specified. Also, the Ethernet system is divided into Fast Ethernet (trademark), Giga Bit Ethernet (trademark) and 10 Gaga Bit Ethernet (trademark) in which the transmission rate is specified at multiples of 10: 100 Mbps, 1 Gbps and 10 Gbps, respectively, according to the circuit capacity. Other communication protocols, e.g., FDDI (Fibre Distributer Data Interface), ESCON (Enterprise System Connection) and FC (Fibre Channel) exist. Not only the signal encoding method but also the transmission rate varies among different communication protocols.
FIG. 10 is a diagram schematically showing a communication system using different communication protocols. SDH/SONET is used for a circuit network connecting buildings, called an intercity communication network. For a LAN used in a building, a communication protocol such as Ethernet (trademark), FDDI, ESCON or FC is used. It is, therefore, necessary for a light transmission device provided at a building inlet to be adapted to each protocol. From the viewpoint of minimizing the investment cost for optical communication devices, there is a demand for a light transmission device capable of operation with both an existing communication protocol and a new communication protocol using one physical layer.
As described above, there is a demand for a multiprotocol-compatible light transmission device. Adapting a light transmission device to different communication protocols and to different transmission rates requires securing a wider-band main signal characteristic for adaptation of the pass band of a main signal line to the range of transmission rates from a low transmission rate to a high transmission rate. Under the present circumstances, the band is limited by the device capacity. Therefore the range of transmission rates realizable on one physical layer is 100 Mbps to 2.5 Gbps, and an applicable communication protocol is determined according to the range.
The performance required of a light receiver for a multiprotocol-compatible light transmission device will be described. In general, the receiving sensitivity (bit error ratio: BER) represents an index for the performance. The receiving sensitivity can be obtained from the ratio of a signal component and a noise component called Q value (see, for example, GovindP. Agrawal “Fiber-Optic Communication Systems”, published from Wiley-Interscience). FIG. 11 is a diagram for the concept of the receiving sensitivity, showing a signal component waveform with respect to time (left-hand side) and a signal component probability distribution (right-hand side). The minimum receiving sensitivity refers to average light input power when the error rate per bit exceeds a certain value and to a threshold value at which the signal level can be correctly determined.
A case where, in a modulated signal in which the probability of occurrence of a level (mark) “1” (mark rate) is ½ in optical communication using a digital communication system, noise distributions with respect to the levels of the mark and “0” (space) are obtained as Gaussian distributions will be considered. The receiving sensitivity BER of a light receiver is expressed by the following equation (1) in which ID represents the discrimination level of the light receiver, I1 the mark-side light intensity, I0 the space-side light intensity, σ1 noise around the mark-side light intensity, and σ0 noise around the space-side light intensity.
                    BER        =                              1            4                    ×                      [                                          erfc                (                                                                            I                      1                                        -                                          I                      D                                                                                                  σ                      1                                        ⁢                                          2                                                                      )                            +                              erfc                (                                                                            I                      D                                        -                                          I                      0                                                                                                  σ                      0                                        ⁢                                          2                                                                      )                                      ]                                              (        1        )            
The coincidence between the error rates on the mark and space sides as shown in equation (2) means that the discrimination level of the light receiver is set to an optimum level such that the code error rate is minimized independently of received light power. In this case, the light receiving sensitivity BER of the light receiver is expressed by equation (3).
                                                        I              1                        -                          I              D                                            σ            1                          =                                                            I                D                            -                              I                0                                                    σ              0                                =          Q                                    (        2        )                                BER        =                              1            2                    ×                      erfc            (                          Q                              2                                      )                                              (        3        )            
From equation (2), the discrimination level ID of the light receiver is expressed by equation (4).
                              I          D                =                                                            σ                0                            ⁢                              I                1                                      +                                          σ                1                            ⁢                              I                0                                                                        σ              0                        +                          σ              1                                                          (        4        )            Equation (4) is substituted in equation (2) to express the Q value as shown by equation (5).
                    Q        =                                            I              1                        -                          I              0                                                          σ              1                        +                          σ              0                                                          (        5        )            
If the extinction ratio of the received light waveform is considered infinite, I0≦0. Further, the mark-side light intensity I1 can be obtained from the average received light power Pin [W] and the conversion efficiency R [A/W]. Noise σ1 around the mark-side light intensity can be expressed by shot noise σs in the light receiving element and thermal noise σT in an electric amplification stage following the light receiving element. Noise σ0 around the space-side light intensity can be expressed by thermal noise σT in the electric amplification stage. As a result, equation (6) is derived.
                    Q        ≈                              RP            in                                                              (                                                      σ                    s                    2                                    +                                      σ                    T                    2                                                  )                                            1                /                2                                      +                          σ              T                                                          (        6        )            Shot noise σs can be expressed by equation (7), and thermal noise σT can be expressed by equation (8). In equations 7 and 8, q represents the amount of charge per electron [C], Id dark current [A] through the light receiving element, Δf the band contributing to noise, kB the Boltzmann constant, T the absolute temperature and RL the load resistance corresponding to the resistance of a feedback resistor of the photoelectric conversion circuit.σs2≈2q(RPin+Id)Δf  (7)σT2≈(4kBT/RL)Δf  (8)
FIG. 12 is a diagram schematically showing the relationship between BER computed by using equations (3) and (6) to (8) and the average received light power with respect to each of transmission rates of 155.52 Mbps, 622.08 Mbps and 2.48832 Gbps. It can be understood from FIG. 12 that BER changes with changes in transmission rate. This discussion presupposes use of the same light receiver and limiting the pass band contributing to noise with respect to each transmission rate. Accordingly, the differences between the BER values shown in FIG. 12 are due to the fact that each of the bands contributing to noise out of the main signal band is cut. Also, the pass band of an electronic device used in the light receiver is sufficiently wide, and the pass band is optimized with respect to each transmission rate by the filtering function of the light receiver. Thus, the band Δf contributing to noise determining shot noise σs and thermal noise σT defined by equation (6) varies.
It can also be understood from FIG. 12 that if the transmission rate is reduced to about ¼, the average input received light power level to provide the same BER is improved by about 3 dB. In general, in an optical fiber transmission line using the ordinary dispersion fiber in the most widespread use heretofore, the distance between light transmission devices can be increased if the minimum receiving sensitivity of the light receiver is lower. It is thought that the transmission distance can be increased if the transmission rate is reduced, and that long-distance transmission becomes difficult if the transmission rate is increased.
However, the threshold value determined in the optical signal loss detection circuit 4 is a value determined with respect to input signal intensity II=R*Pin determined in the numerator of equation (6). That is, the threshold value is expressed by equation (9) below, and is determined independently of the noise terms that determine the value of the receiving sensitivity BER. A in equation (9) is a certain constant. Therefore the threshold value is not influenced by any change in the band contributing to noise when the transmission rate is changed.Ith≈RPin×A  (9)
In the conventional light receiver, the threshold value is fixed with respect to the transmission rate used at the time of initial adjustment. Therefore, when a transmission rate different from that used at the time of initial adjustment is used, the receiving sensitivity BER is changed but the threshold value is not changed. For example, when the transmission rate is changed from 2.48832 Gbps to 622.08 Mbps or 155.52 Mbps in a case where as shown in FIG. 12 the threshold value is set to an average received light power level of about −25.0 dBm corresponding to BER 1×10−5 at the transmission rate 2.48832 Gbps, the operation is error-free such that BER is 1×10−5 or less, but the threshold value does not change by following the transmission rate. Therefore, even if the threshold value is initialized to enable detection of an optical signal break at BER 1×10−5, failure to continue detection of an optical signal break occurs if the transmission rate is changed from the initial value. Thus, the detection accuracy of the optical signal loss detection circuit is reduced and even a communication condition at a receiving sensitivity at which low-error communication can be performed is recognized as the impossibility of communication or circuit failure, resulting in a reduction in operation efficiency of the transmission device and failure to achieve the desired performance.
As described above, the conventional light receiver has a problem that in the case of application of a light transmission device for multiprotocol use the operation efficiency of the transmission device is reduced. That is, different encoding methods are used in different communication protocols and the signal frame pattern and the transmission rate vary, resulting in failure to detect a signal break in an optical signal by following the receiving sensitivity.